Optical switching, multiplexing, and demultiplexing have been accomplished in the past by using an interconnection apparatus having a plurality of closely spaced input waveguides communicating with the input of a star coupler. The output of the star coupler communicates with an optical grating comprising a series of optical waveguides, each of the waveguides differing in length with respect to its nearest neighbor by a predetermined fixed amount. The grating is connected to the input of a second star coupler, the outputs of which form the outputs of the switching, multiplexing, and demultiplexing apparatus. An example of such an interconnection apparatus is disclosed in U.S. Pat. Nos. 5,002,350 and 5,136,671.
FIG. 1a illustrates a prior art optical interconnection apparatus. The geometry of such an apparatus may be such that a plurality of separate and distinct wavelengths each launched into a separate and distinct input port of the apparatus will all combine and appear on a predetermined one of the output ports. In this manner, the apparatus performs a multiplexing function. The same apparatus may also perform a demultiplexing function. In this situation, a plurality of input wavelengths launched into one of the input ports is separated from each other and each wavelength is directed to a predetermined one of the output ports of the apparatus.
The apparatus of FIG. 1a may also be employed as a wavelength router in an optical network so as to efficiently drop and add optical channels at various cross connect points. A particular channel may have to pass through several routers without regeneration before leaving the network. Thus, a maximally flat passband is desirable for each router. The required flatness then depends on the largest number of routers that are concatenated. For instance, the passband ripple for each router should preferably stay below a few tenths of a decibel, if more than 10 filters are concatenated. Thus there is a need for an arrangement for obtaining a relatively small ripple over a wide passband.
One approach for achieving a wider passband is described in the U.S. Pat. No. 5,412,744, issued May 2, 1995 for a "frequency Routing Device Having a Wide and Substantially Flat Passband." Basically, a Y-branch coupler is connected to the remote ends of two adjacent waveguides, wherein the waveguides are located a predetermined distance apart to produce a given passband width. As will be explained in more detail below, the Y-branch coupler allows for a substantially wider passband. However, there are certain disadvantages with employing the Y-branch coupler. For example, because optical signal traveling through a Y-branch generates unwanted transmission components, the length of the Y-branch coupler must be long enough so that the undesirable components do not reach the wavelength router. Furthermore, the gap between the two branches of the Y coupler is in the order of 3-5 microns. The relatively long length and substantially small gap between the branches lead to undesired fabrication errors, which are difficult to overcome.
Thus, there is a need for a wavelength router that has a substantially wide passband, without the disadvantages of prior art systems.